The effect of four synthetic analogues of the 37-residue winter flounder type I antifreeze protein (AFP), which contain four Val, Ala or Ile residues in place of Thr residues at positions 2, 13, 24 and 37 and two additional salt bridges, on the binary lipid system prepared from a 1:1 mixture of the highly unsaturated DGDG and saturated DMPC has been determined using FTIR spectroscopy. In contrast to the natural protein, which increases the thermotropic phase transition, the Thr, Val and Ala analogues decreased the thermotropic phase transitions of the liposomes by 2.2 degrees Celsius, 3.4 degrees Celsius and 2.4 degrees Celsius, while the Ile analogue had no effect on the transition. Experiments performed using perdeuterated DMPC showed that the Ala and Thr peptides interacted preferentially with the DGDG in the lipid mixture, while the Val peptide showed no preference for either lipid. The results are consistent with interactions involving the hydrophobic face of type I AFPs and model bilayers, i.e. the same face of the protein that is responsible for antifreeze properties. The different effects correlate with the helicity of the peptides and suggest that the solution conformation of the peptides has a significant role in determining the effects of the peptides on thermotropic membrane phase transitions.